1. Field of the Invention
The invention discloses an adsorbent of predetermined composition comprising chitosan and one or more additional materials enabling removal of certain compound(s) from a fluid. A chitosan based adsorbent is functionalized and cross linked to a predetermined degree to enhance its capacity and selectivity for various molecules and/or ions; additional ligands may be added to enhance overall adsorption capacity.
2. Description of Related Art
The literature contains hundreds of references (Mourya, 2008) to chitosan and its ability to adsorb various heavy metals. Unfortunately no one has solved two key chitosan problems critical to commercialization for water purification. As conventionally synthesized chitosan is a fragile hydrogel containing about 90% water; chitosan beads, as a hydrogel, are too mechanically fragile to withstand commercial environments. Secondly, few researchers have explored low cost synthesis approaches, including low cost ligands, capable of metal removal at concentrations of interest, such as selenium at or below 25 ppb.
The positive features of chitosan are that it is abundant, non-toxic, relatively inexpensive and exhibits excellent adsorption capacities for metals. Almost all published work done on chitosan is as a hydrogel. The instant invention discloses a chitosan material with sufficient physical strength and metal removal capacity to be commercially attractive.
Metal removal with conventional metal oxide adsorbents to establish the “state-of-the art” is presented. Various methods have been published for processing chitosan into a hydrogel; Merrifield, 2004; Boddu, 2008; Li, 2006; Sabarudin, 2006 are examples.
Portier in U.S. Pat. No. 4,882,066 teaches coating a porous solid with a multi-micron thick film of chitosan. Portier demonstrated removal of various metals from streams; however Portier did not disclose or suggest the need for a crosslinking agent or compound or a ligand for metal binding Like Boddu, Portier did not consider or suggest the benefit of sub-micron particles in combination with chitosan and additional compounds.
Boddu, U.S. Pat. No. 6,786,336, teaches a “composite chitosan biosorbent” comprising a ceramic support, twice coated with a chitosan gel material. Boddu teaches an ultra fine ceramic of alumina or silica having a particle size from about 10 to about 150 microns. Boddu teaches to acid wash, rinse and then dry the ceramic prior to application of a first chitosan gel, followed by drying and application of a second chitosan coating and then drying; the final product being about 21% chitosan by weight. Good adsorption is shown for chromium (VI), Boddu, 2003, in sulfate and chloride at chromium concentrations above 20 ppm.
As taught by Prashanth (2006), chitosan undergoes radical-induced depolymerization in the presence of potassium persulfate at 60° C., leading to extensive crosslinking of the fragmented chains on subsequent cooling at 4° C. As a result, a possible conformational change leading to higher crystallinity, is observed. In crosslinked chitosan, the polymeric chains are interconnected by crosslinkers, leading to the formation of a 3D network. They can be formed by complexation with another polymer, generally ionic, or by aggregation after chitosan grafting. Crosslinkers are molecules of molecular weight much smaller than those of the chains between two consecutive crosslinks. Other components such as additional polymers to form a hybrid polymer networks (HPN) or semi- or full-interpenetrating polymer networks (IPN) can be added during the crosslinking reaction. The biocompatibility of such modified chitosans has not yet been assessed, due to the presence of traces of potentially toxic auxiliary molecules or crosslinkers, whose administration in humans may be problematic. To date, the most common crosslinkers used with chitosan are dialdehydes such as glyoxal and in particular glutaraldehyde. However, the main drawback of such reactions is that they are generally considered to be toxic. For example, glutaraldehyde is known to be neurotoxic, its fate in the human body is not fully understood and glyoxal is known to be mutagenic. Therefore, even if products are purified before administration, the presence of free unreacted dialdehydes in the products can not be completely excluded. Besides dialdehydes, crosslinkers such as diethyl squarate, oxalic acid or genipin can exhibit direct crosslinking mechanisms, although they remain incompletely elucidated. Crosslinked chitosan can also be formed by direct interaction between polymeric chains, without the addition of crosslinkers. An example is crosslinked chitosan, which was formed as a byproduct of persulfate-induced free radical graft copolymerization.
U.S. Pat. No. 7,354,600 and Kast 2001 teach a thiolated polymer, optionally chitosan, with favorable mucoadhesive properties for in-vitro drug delivery.
There is a need for a low cost adsorbent effective on effluent streams of industrial origin and also applicable to water purification in general.